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Nitrogen functionalized biochar as a renewable adsorbent for efficient CO2 removal Hanieh Bamdad, Kelly A Hawboldt, and Stephanie L. MacQuarrie Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03056 • Publication Date (Web): 30 Oct 2018 Downloaded from http://pubs.acs.org on October 31, 2018
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Nitrogen functionalized biochar as a renewable adsorbent for efficient CO2 removal Hanieh Bamdad a*, Kelly Hawboldt a, Stephanie MacQuarrie b a
Department of Engineering and Applied Science, Memorial University, St. John’s, NL, Canada, A1B 3X9 b
Department of Chemistry, Cape Breton University, Sydney, NS, Canada, B1P 6L2
Graphical abstract
Abstract In this study, biochar was thermally and chemically (thermo-chemically) modified and compared to the unmodified parent char in carbon dioxide adsorption. The biochars were sourced from sawmill residues and produced via fast pyrolysis in an auger reactor. The biochar was chemically functionalized using two novel methods of amine functionalization: i) nitration followed by reduction and ii) condensation of aminopropyl triethoxysilane on the surface. The obtained outcomes indicated functionalization resulted in a reduction in the pore volume and surface area of the biochar. The biochars (unmodified and chemically modified) were thermally activated via air diluted with nitrogen at moderate 560 ºC to determine if the adsorption properties could be enhanced. The thermally treated functionalized chars had a lower H:C ratio, higher surface area, micropore volume, and sufficient amount of nitrogen compared to the unmodified char. The thermally treated aminopropyl triethoxysilane char had the highest adsorption capacity of 3.7
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mmol/g with 0.24 wt% nitrogen. Biochars sourced from residues demonstrated high efficiency of carbon dioxide removal, comparable to some synthesized adsorbents reported in the literature. Keywords: Biochar; Adsorption; Functionalizing; Thermal activation; CO2 Introduction Sawmill residues in the form of bark, sawdust and saw chips are currently stockpiled and represent a safety and environmental liability; this is particularly problematic in remote regions where transport of this material offsite is costly. Thermal conversion of this biomass via pyrolysis 1, to bio-oil 2 and/or biochar, is one method of monetizing these residues. Biochar can be used as a soil amendment, adsorbent for contaminants in water, wastewaters 3, and gas
4,5,
among others. The removal of acid gases such as H2S and CO2 from gas streams (such as vent/flare gases) is one such application. Traditional methods to remove these gases can be energy and space intensive, may require expensive and/or toxic chemical, and complex infrastructure 6. In offshore and any other remote locations (e.g. landfills, small wastewater treatment plants etc.) small scale and less operationally intensive method for gas treatment are required. The common method for acid gas removal is absorption, the acid gases are removed using solvents such as monoethanolamine (MEA) and diethylamine (DEA). Although the selectivity of this form of separation is relatively high, amines are corrosive and highly volatile and the method is cost-intensive due to high energy needs for solvent regeneration (around 85 kJ/mol CO2)
7
and space requirements (separate column for regeneration). Adsorbent systems
using porous solids are an attractive alternative to traditional gas-liquid contacting systems. Adsorbents sourced from waste biomass is potentially a more sustainable approach to gas treatment, however factors such as adsorption efficiency, needs of the operator (e.g. bulk removal vs. high purity gas products), regeneration and disposal options must be considered. In order to assess these factors experiments are required to determine adsorption capacities, regeneration potential, and stability of the spent adsorbent. Biochar-based adsorbents, sourced from forestry residues and produced via fast pyrolysis, are a potential alternative to traditional solid CO2 adsorbent systems. CO2 was chosen as target since it is often associated with H2S in petroleum and landfill gases, and can serve as a surrogate for H2S (a safety and environmentally problematic gas). However, there is potential to improve the adsorbent characteristics by 2 ACS Paragon Plus Environment
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chemically and/or thermally activating the biochar. Incorporating nitrogen functional groups into carbon-based adsorbents enhances surface basicity and could improve adsorption of particular compounds (e.g. H2S and CO2) and/or the added nitrogen can chemically interact with these acidic gases, i.e. dipole–dipole, hydrogen bond, covalent bond, etc. 8. Another motivation for selecting biochar as an adsorbent was low cost/availability even after additional functionalization steps relative to other adsorbents. The biochar price is only 1/6 of that of commerce activated carbon 9. There are several methods for functionalizing carbon surfaces with nitrogen groups. Ammonia is commonly used
10
where adsorbent particles are placed in a tube furnace. The adsorbent is
gradually heated up to the specified temperature typically with N2 purging. Once the set point temperature is reached, the N2 is replaced with NH3 or NH3 mixture. Zhang et al.
11
modified
soybean straw biochar by NH3 over a temperature range of 500-900 ºC. This not only enhances the surface area (from 1.5 to as high as 496 m2/g), but also increased the CO2 adsorption capacity up to 1.8 mmol/g. Other methods of introducing nitrogen include addition of nitrogen rich proteins and amino acids. Jayshri et al.
12
synthesized nitrogen enriched carbon using local
soybean as the nitrogen source (soy protein) followed by chemical activation using zinc chloride and physical activation using CO2 . The surface area of synthesized nitrogen enriched carbon increased to 811 m2/g and the breakthrough adsorption capacity to 0.5 mmol/g at 120 °C. Pevida et al.
13
applied different alkylamines to activated carbon (Norit CGP) through a wet
impregnation method to increase the basicity and nitrogen content. The impregnation decreased the surface area (from 1762 to 90 m2/g) and there was no increase in adsorption capacity. In fact, the raw activated carbon showed the highest CO2 adsorption capacity. It was proposed the amine might block a fraction of the pores, reducing the surface area for adsorption. In order to enhance CO2 adsorption capacity, Zhang et al.
11
used CO2 activation at high temperatures (500-900 ºC)
on the soybean straw based biochar. The surface area of the aminated modified chars increased from 5.5 to 397 m2/g and the CO2 adsorption capacity at 30 ºC increased from 1 to 1.7 mmol/g with increasing activation temperature (500 to 800 ºC). Further increasing activation temperature to 900 ºC resulted in a decrease in adsorption capacity to 1.5 mmol/g. The decrease could be a result of thermal degradation of some amine functional groups, indicating an optimum activation temperature(s) to maximize adsorption. 3 ACS Paragon Plus Environment
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Thermal activation of biochar has focussed on high temperatures (up to 900 ºC), however at high temperatures the role of the nitrogen functional groups in adsorption is partially or completely lost. As such, in this work we focussed on activating the biochars in an air or oxygen environment at moderate temperature (below 600 °C) in order to both achieve higher surface area while minimizing functionalization loss. The biochar was produced from fast pyrolysis of local softwood residues in an auger reactor. Two novel methods to introduce nitrogen functionality to the biochars were used. A subset of the biochars was thermally activated using a diluted air-nitrogen mixture at a moderate temperature (560 ºC) and compared with nonactivated chars. The relationship between the impact of porous structures and nitrogencontaining group on upgrading CO2 adsorption capacity of biochar was assessed. Further investigation on functionalizing of these biochars can enhance the adsorption and allow the chars to be “tailored” to a target gas (such as other acid gases, H2S). 1. Materials and Methods 1.1. Materials A commercial chemically activated wood-based carbon (Norit CA1) from Sigma-Aldrich was used to compare against the biochars. Sexton Lumber sawmill (Bloomfield, Newfoundland and Labrador) supplied the softwood sawmill sawdust feedstock. All chemicals utilized in the functionalizing sector were reagent grade chemicals purchased from Sigma-Aldrich and Fischer Scientific. The chemicals used for functionalizing were sulphuric acid, nitric acid, 2-propanol, ammonium hydroxide, sodium hydrosulfite, acetic acid, aminopropyl triethoxysilane, hydrochloric acid, and ethanol. 1.2.
Adsorbent Preparation
The sawdust was dried for 2 days at ambient temperature to decrease the moisture to ~0.12 g/g (12 %). The average particle size of the samples was reduced to less than 2 mm after grinding and then dried again at 70 °C overnight to further decrease the moisture content to 2 % prior to pyrolysis. Fast pyrolysis (at 500 oC) was used to produce the biochar in an auger reactor. Details on the fast pyrolysis reactor is reported elsewhere 14. The biochar samples are labelled according to the type of activation/functionalization, temperature of pyrolysis and, when required, temperature of activation. 4 ACS Paragon Plus Environment
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The biochars were aminated based on a modified literature method
15
comprised of two steps.
The first step is nitration of the biochar. Concentrated sulphuric acid (18 M, 50 mL) was added slowly to concentrated nitric acid (15.7 M, 50 mL) at 0 oC. A 9 g sample of washed biochar was added to the acid mixture and stirred for 50 min. The mixture was filtered and washed with distilled water and 2-propanol. The residue was then air-dried at ambient temperature resulting in the introduction of nitro groups to the surface of the biochar. The nitrated biochar was then reduced by addition of 50 mL distilled water and 20 mL of ammonium hydroxide (3 M) and stirred for 10 min without heating. Sodium hydrosulfite (28 g) was added to the solution and allowed to mix overnight under a reflux condenser to avoid solvent evaporation. 20 mL of glacial acetic acid (17.5 M) was diluted in 100 mL water and added to the solution and stirred for 5 h. The solution was then cooled to room temperature, filtered, washed with distilled water and 2propanol, and air-dried. The final product is aminated biochar and the samples’ name was prefixed with “AM” in the text. Figure 1 outlines the synthesis.
Fig. 1. Schematic example of the nitration and reduction of biochar 15 under reflux (exothermic) Aminopropyl triethoxysilane (APTES) was grafted to the surface of the biochar by suspending biochar in distilled water in a 10:1 ratio (char/water) and slowly adding APTES (20% by weight). The APTES-biochar solution was sonicated for 10 min. To promote the condensation reaction, the pH of the suspension was then adjusted between 3 and 4 using concentrated hydrochloric acid (11.7 M) and allowed to sit for 1 h at ambient temperature, and condensed (refluxed) over 6 h at 70 ⁰C. The resulting biochar was filtered and washed with ethanol followed by distilled water, and dried under vacuum at 40 ºC overnight. The biochar produced is labeled with “AP” (Figure 2). 5 ACS Paragon Plus Environment
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OH
OH
OH OH C OH O
O
O OH Si
(EtO)3Si(CH2)3NH2 EtOH heat
NH2
O C OH O
O
OH
OH
(1)
(4) AP
Fig. 2. Surface modification of biochar with APTES 16–18 under reflux at 70 °C Samples of the biochars were thermally activated at 560 °C using air flow diluted with nitrogen (5% oxygen) for two hours at a 100 mL/min flow rate and labeled as “A-560”. The biochar samples were heated up gradually to the specified temperature in the tube furnace with N2 purging and once the set point temperature is reached, the N2 is replaced with air-nitrogen mixture.
1.3. Adsorbent Properties The microstructure of the biochars was obtained using a scanning electron microscope (SEM) (FEI 650F). Samples were mounted on carbon adhesive 12 mm diameter tabs, which were put on aluminum stubs using carbon tape to avoid the formation of an electric charge on the surface during scanning. Images were taken at low vacuum, with a pressure of 93.3 Pa. The instrument has a secondary-electron (SE), a backscattered-electron (BSE), and a mix of (SE) and (BSE) imaging modes for morphological analyses of the samples. Textural properties of all samples were determined by N2 adsorption-desorption isotherms obtained at 77 K with automatic equipment (Micrometrics Tristar II Plus, USA). Prior to measurement, the flowing-gas degassing was employed at 200°C over night which removes adsorbed contaminants from the surface and pores of the samples. The average pore size and micropore volume were measured via the pore size distribution technique, BJH (Barrett-Joyner-Halenda) and the t-plot method, respectively. The BET (Brunauer–Emmett–Teller) was used to calculate the surface area of the biochar. The bulk elemental analysis of the biochar was performed using a CHN elementary analyzer (Perkin 6 ACS Paragon Plus Environment
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Elmer Series II 2400). Infrared spectra were determined by using a FTIR (Bruker Alpha FTIR spectrometer, accessory type: Single-bounce diamond crystal ATR) with a range of 400 to 4000 cm-1 and a total of 24 scans for both background and sample measurement.
1.4. Adsorption-desorption experiments in a fixed bed reactor A single-bed adsorption unit was made from borosilicate glass for conducting the adsorption experiments. Figure 3 illustrates the schematic of the adsorption-desorption setup.
Fig. 3. Schematic of lab-scale adsorption-desorption system Before each experiment, the adsorbent was dried in the oven at 60 ºC overnight. Prior to the analysis, the samples were degassed at 150 °C by purging N2 flow through the adsorption column for 1 hour and then cooled to room/desired temperature. Drying and degassing steps make us confident that all the moisture and adsorbed matters were removed and subsequently the impact of water vapour on the adsorption can be eliminated. Approximately 2.0 g of biochar was placed in the fixed bed reactor (length: 300mm, ID: 15mm), and pure CO2 at 60 mL/min was introduced into the reactor. The adsorption experiments were conducted at room temperature (20 °C). The flow rate of CO2 was controlled with mass flow controller and the composition of the outlet gas stream was continuously monitored with a gas analyzer (OXYBABY® M+). The process was terminated when the bed was saturated as measured by CO2 detected at exit (break through). The adsorption capacity was calculated by integration of the area below the 7 ACS Paragon Plus Environment
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breakthrough curves
19
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(equation 1) which is determined by the ratio of outlet to inlet adsorbate
gas concentration as a function of time. 𝑡
𝑄=
𝐹∫0(𝐶0 ― 𝐶)𝑑𝑡
(1)
𝑚
where Q is adsorption capacity (mmol/g), F is flow rate of inlet CO2 (mL/min), C0 is concentration of inlet CO2 (mmol/L), C is concentration of outlet CO2 (mmol/L), and m is weight of the biochar (g). The desorption experiment was done using N2 at 100 mL/min and ambient temperature. Nitrogen was flowed through the system and again CO2 measured at the exit. The spent biochar after regeneration was then reused in the CO2 adsorption experiment (CO2 at 60 mL/min).
2. Results and Discussion 2.1. Characterizations Physical, chemical, and morphological properties of raw and modified biochars are summarized in Table 1. Table 1. Properties of biochar samples and activated carbon Surface Avg. Micropore C H N Area Pore Samples volume (wt%) (wt%) (wt%) size (BET) (cm3/g) (nm) (m2/g) AM-SW500 3.22 7.20 N/A 61.99 2.56 3.90
H:C
N:C
0.04
0.063
AP-SW500
59.18
3.89
0.026
74.58
2.55
0.30
0.03
0.004
SW500
95.58
4.36
0.033
76.37
2.36
0.15
0.03
0.002
SW500-A-560
391.76
3.12
0.159
77.24
1.90
0.12
0.025
0.002
343.32
2.97
0.133
68.37
1.46
3.17
0.021
0.046
394.12
3.08
0.160
80.15
1.63
0.24
0.020
0.003
1166.49
3.63
0.325
81.34
2.10
0.28
0.026
0.003
AM-SW500-A560 AP-SW500-A560 AC (Norit)
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The BET surface areas for the biochars produced in this study ranged from 3 (aminated char) to 394.1 (thermally activated modified biochar) m2/g. The commercial activated carbon (Norit) has the highest surface area at 1166.5 m2/g. The pore volume of the functionalized samples (Table 1row 1 and 2) decreased compared to the other chars. This indicates the amine molecules may be “blocking” smaller pores, thereby reducing surface area, which has been reported by others during functionalization of porous materials
20,21.
The textural properties of the samples were
further developed by thermal activation, which increased the surface area (~3 times) and pore volume of the product (Table 1- row 4-6). This occurred due to thermal degradation and volatilization processes 21. The impact of activation on the functional groups is discussed below through FTIR analyses. The nitrogen content increased following amine functionalization for both methods (Table 1). In biochar without addition of nitrogen groups, lower H:C ratio indicates a hydrophobic char that can favour adsorption of nonpolar molecules (such as CO2). However, this trend was not noted in the aminated chars as will be discussed in more detail in subsequent sections. The carbon content increased, while the nitrogen and hydrogen decreased during the activation process for both functionalized samples, indicating the degradation of some functional groups 22,23. The reduction of the H:C ratio after heat treatment has the potential to increase the adsorption capability of the biochar. The highest surface area with the lowest H:C was in the activated aminated char, APSW500-A-560. To determine the impact of functionality, the FTIR spectra of all samples were analyzed in Figure 4.
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N-H/C=C
C=O N-H SW500 SW500-A-560 AM-SW500
C-N
AM-SW500-A-560
N-H
AP-SW500 AP-SW500-A-560
C-N Si-OR 3900
3400
2900
2400 1900 1400 Wavenumber (cm-1)
900
400
Fig. 4. FTIR analysis of different biochar samples The identification of nitrogen functional groups in IR diagrams can be challenging, as they are present in the same wavelength as other functional groups and can be masked. The absorption peak at 900-660 cm-1 is likely N-H bending, as it was found in all N-functionalized biochar samples and are visible in nitrogen functionalized chars but not in the unmodified biochar samples. C-N groups were observed at 1250-1000 cm-1, more predominantly in amine functionalized samples due to higher nitrogen content. The peaks in the range of 1000 to 1200 cm-1 were present only in the APTES chars (AP-SW500, AP-SW500-A-560), could indicate SiOR. This was expected due the (EtO)3Si(CH2)3NH2 used in the ATPES process (Figure 2). All chars showed identical peaks at 1650-1550 cm-1 likely corresponding to C=C and/or N-H bending. The C-N and N-H peak intensities for aminated samples (AM-SW500, AM-SW500-A560) were strongest due to higher loadings of nitrogen. The peak appeared in the range of 1700 cm-1 could be related to C=O (Carboxyl group). The carboxyl group could be displaced by amide after functionalizing and therefore, the C=O peak was less prominent for aminated samples (Figure 5). Phenol functional groups (O-H) were observed as small peaks in the range of 13901310 and 3900-3300 cm-1. The absorption peak intensities decreased for activated samples (dashed lines) likely due to loss of some functional groups. For instance, the intensity of C-N functional group in AM-SW500 reduced after activation. The impact of N-functional groups and 10 ACS Paragon Plus Environment
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decomposition after activation on CO2 adsorption process will be outlined in subsequent sections. SEM analyses on the fresh (SW500), aminated, and activated surfaces are shown in Figure 5 in order to study the appearance effects of functionalizing and activation on biochar samples.
b
c
d
e
f
AM-SW500
SW500
a
AP-SW500
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h
i
j
k
l
AM-SW500-A-560
SW500-A-560
g
AP-SW500-A-560
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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Fig. 5. SEM images at different resolutions (Best mode was selected for each), left column (low resolution: 300-500µm), right column (high resolution: 30-100µm) The overall morphology of the samples (Fig. 5a,c,e) at low resolution instrument (i.e., at large length scales) reveals no marked differences between samples. At high resolution (Fig. 5d,f), the porous structure of the samples is partly diminished after functionalizing, indicating the amine groups were distributed unevenly and occluding some of the pores. The reduction in the surface area and pore volume of functionalized chars validates the SEM results
24.
After one step
physical activation, the carbon framework was observed more clearly and the pores became developed and broadened in both low and high resolutions (Fig. 5g,h,i,j,k,l). The etching action
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between the walls and the activating agent (oxygen diluted with nitrogen) at a high temperature as a result of pore skeleton development which leads to more large-volume pores 25. 2.2.
CO2 Adsorption-desorption
As indicated above, our ultimate goal is to study the adsorption of acidic/sour gases (H2S and CO2). CO2 is used as a surrogate for both in these screening experiments, as it does not have the safety issues associated with H2S. We have also shown through molecular modeling that the affinity of the chars for CO2 is on the same order of magnitude as H2S. The impact of amine modification on CO2 adsorption capacity was studied at maximum adsorption capacity conditions (20 °C, 60 mL/min, pure CO2). The adsorption performance for all samples and breakthrough curves of two samples (original and modified) are presented in Figure 6.
Fig. 6. Comparison of maximum adsorption capacity of biochars at 20 °C, inlet feed flow rate of 60 mL/min, and pure CO2; breakthrough curves: green for SW500 and blue for APSW500-A-560
As the nitrogen loading increases, the adsorption capacity decreases likely due to blocking of pores and/or coating the adsorbent surface by the larger amine groups, preventing CO2 diffusion on to the pores
26,27
particularly at low temperature
28.
For instance, among functionalized
samples, AP-SW500 showed a higher adsorption capacity in spite of the lower nitrogen loading 13 ACS Paragon Plus Environment
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relative to the AM chars. This result is consistent with the characterization tests such as BET surface area and elemental analysis as discussed in the previous section. To further increase the surface area and promote CO2 adsorption, the samples were activated by diluted airflow for two hours. The CO2 adsorption was lower for the two sets of functionalized samples compared to non-functionalized char, but higher after the activation step. For AM-SW500, the adsorption after functionalization with nitrogen is 1.8 mmol/g with 39 mg N/g; while, after activation, the CO2 adsorption capacity rose to 3.4 mmol/g. This indicates there is a balance between functionality and surface area when adding groups (such as amine) to enhance CO2 adsorption 29. Further testing is required to determine the optimum(s) nitrogen loading and assess the impact of the thermal treatment on the nature of the nitrogen and other functional groups. The nitrogen amounts decreased from 18-20% in the activation process of the biochars (Table 1). In addition to nitrogen loss, there was a decrease in the nitrogen functional peaks in the FTIR. The nitrogen loss through volatization and decomposition of the nitrogen functionality is likely the reason for the peak reduction. Comparing the activated N-loaded biochars (AP-SW500-A-560 and AM-SW500-A-560) to the commercial carbon (Norit), the overall SA is lower, but they demonstrate enhanced adsorption. The reason could be due to a trade-off between the textural and chemical properties; that is, even at lower SA and pore volume the added functionality enhances the adsorption via chemical interaction between the adsorbate and the amines
30
and the more hydrophobic surface. After
activation, the adsorption for the N-functionalized chars is almost the same, while the nonfunctionalized char is lower. As there was no loss of inherent functional groups for the nonfunctionalized chars and an almost equivalent increase in SA for all chars, this shows that the nitrogen groups are playing a role in adsorption. The activated material, AP-SW500-A-560, with a surface area of 394 m2/g and 0.24 wt% N, was the best adsorbent tested (3.7 mmol CO2/g). Table 2 summarizes the prepared biochar and other carbon based adsorbents in the literature including templated carbons and chemical activated adsorbents. Table 2. Summary of comparison between prepared sample and other adsorbents Sorbents
Feedstock
Activation agent, Temp.(°C)
CO2 Capacity (mmol/g) 14
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Experimental Conditions (T,P,%CO2,F)
Proces s Scale
Ref.
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N-doped Microporous Carbon N-doped Activated Carbon
Urea formaldehyde resin
KOH, 500-800
Bean dreg
KOH, 600-800
1.8-3.76
25 °C, 1atm,100, 30mL/min
Lab
[23]
3-4
25 °C, 1 atm, 100, N/A
Lab
[24]
Lab
[25]
Zeolite
N/A
4
25 °C, 1 atm, 100, 50mL/min
N-doped porous carbons
Polyimine
KOH, 600-750
2-3.1
25 °C, 1 atm, 100, N/A
Lab
[26]
UltraMicroporous Carbons
Cyanopyridiniu m dicationic salt
N/A
3.68
25 °C, 1 atm, 100, N/A
Lab
[27]
AP-SW500A-560
Sawdust softwood
3.2-3.7
25 °C, 1 atm, 100, 60mL/min
Lab
This work
N-doped template carbon
Air, 560
The functionalized, activated char showed adsorbent capacities on par with or exceeding those of other commercial or synthetic adsorbents. The advantage with this char is in addition to producing the char; fast pyrolysis of forestry residues produces oil with energy and high value chemical potential applications 31. The stability of the samples AP-SW500-A-560 and AM-SW500-A-560 was studied by a series of adsorption and subsequent regeneration (using N2 at room temperature) cycles. Regeneration experiments are typically conducted at high temperature (ranging from 100-500 °C), since these temperatures accelerate the desorption process
32.
At this stage of the study, we used room
temperature to regenerate the char in an effort to assess the binding of CO2 at these conditions. The reasons were two fold, i) to assess the spent biochars use in soils and understanding the CO2 sequestration capacity of the char at ambient conditions is more relevant ii) to decouple the change in char structure from the impact of temperature so we can assess impacts of cycling. Figure 7 illustrates the impact on adsorption capacity as a function of regeneration. 15 ACS Paragon Plus Environment
Energy & Fuels
4 AP-SW500-A-560
Adsorption capacity (mmol/g)
3.5
AM-SW500-A-560
3 2.5 2 1.5 1 0.5
Cy cl e
10 th
Cy cl e
9t h
Cy cl e
8t h
Cy cl e
7t h
Cy cl e
6t h
Cy cl e
5t h
Cy cl e
e
4t h
Cy cl
cl
3r d
Cy
2n d
Cy cl
e
e
0 1s t
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Fig. 7. CO2 adsorption capacity of cyclic adsorption-desorption experiments After three cycles, the regeneration capacity is slightly decreased. By five cycles, the adsorption capacity has decreased by 4-8% and by ten cycles, the decrease is 20%. In another study
13,
where three cycles were done on nitrogen-enriched carbons for CO2 capture, the decrease in capacity was 5-20% depending on the nature of the nitrogen groups. The regeneration in this case was done under vacuum and 25 ºC. The FTIR analyses of the “regenerated” biochar indicated that a small percentage of CO2 remains on the surface (likely due to chemisorption). This was corroborated by desorption tests in (3Flex surface characterization analyzer MicroMeritics) which showed some CO2 remains on the structure after regeneration. This could account for the decrease in adsorption capacity as in this experimental system the outlet CO2 is measured. This also observed elsewhere
13.
Our work shows that the modified biochar shows
good regeneration potential however, more studies are required to determine the strength of the CO2 binding (e.g. higher temperatures and/or lower pressures in desorption). Conclusion In this work, N-functionalized biochars were prepared and CO2 adsorption experiments were conducted comparing both functionalized and non-functionalized chars. Non-activated functionalized biochars adversely affecting CO2 adsorption. However, moderate thermal 16 ACS Paragon Plus Environment
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Energy & Fuels
activation enhanced the SA and retained enough functionality to generate a material capable of adsorbing CO2 efficiently. After thermal treatment, there was a decrease in the nitrogen content, indicating possible decomposition of some N-containing functional groups and loss of nitrogen. However, thermal activation of the functionalized chars led to higher surface area, pore volume, and lower H:C ratio and ultimately N-enriched biochar followed by moderate physical activation (AP-SW500-A-560) was found to have much higher adsorption capacity compared with commercially available activated carbon (Norit CA1) and recent carbon-based adsorbents in the literature. It appears that retaining some nitrogen functionality enhances adsorption and makes up for a decreased SA limiting the physical adsorption. This study reports the use of moderate, rather than extreme activation temperatures, combined with tailored functionalization of readily available and sustainably sourced biochar as an alternative to more costly adsorbents. Further investigations should focus on optimization of activation conditions, nitrogen loading, and desorption conditions to evaluate the impact of the thermal treatment on the nature of functional groups responsible for chemical adsorption.
Acknowledgment This work was carried out with the support of NSERC (Natural Science and Engineering Research Council of Canada), SGS (School of Graduate Studies of Memorial University), and BioFuelNet Canada. The authors sincerely acknowledge the valuable assistance provided by Dr. MacQuarrie’s group (Cape Breton University) in functionalizing studies conducted during this work and Dr. Sadegh Papari (Memorial University) for producing the biochar samples. References (1)
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